Porous ceramic honeycomb structures are widely used as catalytic converter substrates and/or diesel particulate filters. The earliest cordierite ceramic honeycombs for these applications consisted of reaction-sintered cordierite substrates prepared from extruded mixtures of talc, alumina, and kaolin such as described in U.S. Pat. No. 3,885,977, for example. These were found to have suitable chemical durability, inertness, refractoriness, and thermal shock resistance for the application, and were generally cost effective to manufacture. For purposes of the present description the term “cordierite” used alone refers to orthorhombic cordierite (orthorhombic Mg2Al4Si5O18 and its solid solutions), indialite (hexagonal Mg2Al4Si5O18 and its solid solutions), and mixtures thereof unless expressly stated otherwise.